1. Field of the Invention
The present invention relates to a cell pack of a direct methanol fuel cell for use as power of portable electronic devices, and more particularly, to an air breathing direct methanol fuel cell pack with an effective air supply unit and a reaction byproduct exhaust unit.
2. Description of the Related Art
A direct methanol fuel cell (DMFC), which generates electrical power by electrochemical reactions between methanol as fuel and oxygen as an oxidizing agent, has a high energy density and a high power density. Also, since the DMFC uses methanol directly as fuel, external peripheral devices such as a fuel reformer are not required and the fuel is easily stored and supplied. Further, a monopolar DMFC can be operated at room temperature and atmospheric pressure and can be made lightweight and miniaturized, thus having very wide applications including mobile communications equipment such as mobile cellular phones, PDAs or laptop computers, medical appliances, military equipment and so on.
As described above, DMFCs produce electricity by electrochemical reaction between methanol and oxygen. A single cell of such DMFCs is constructed such that an electrolyte membrane is interposed between an anode and a cathode.
Both of the anode and cathode include a fuel diffusion layer for supply and diffusion of fuel, a catalyst layer at which electrode reactions, that is, oxidation/reduction of fuel, occur, and electrode backings. As the catalyst layer for oxidation/reduction, precious metals having good characteristics even at low temperatures, such as platinum (Pt), are used, and alloys of transition metal such as ruthenium (Ru), rhodium (Rh), osmium (Os) or nickel (Ni) can also be used for preventing catalytic poisoning due to reaction byproducts, e.g., carbon monoxide. Carbon paper or carbon cloth is used as the electrode backings, and the electrode backings are waterproof for easy supply of fuel and easy exhaustion of reaction products. The polymer electrolyte membrane has a thickness of 50 to 200 μm. A proton exchange membrane having ionic conductivity is usually used as the electrolyte membrane.
The following reaction equations occur in the anode where fuel is oxidized and the cathode where oxygen is reduced, respectively.
[Anode Reaction]CH3OH+H2O→CO2+6H++6e−
[Cathode Reaction]3/2 O2+6H++6e−→3H2O
[Overall Reaction]CH3OH+3/2O2→2H2O+CO2
In the anode, carbon dioxide, six protons and six electrons are generated by reaction between methanol and water, that is, oxidation, and the generated protons are transferred to the cathode via the proton exchange membrane. In the cathode, protons and electrons supplied from an external circuit react with oxygen to produce water, that is, reduction. Thus, the overall reaction corresponds to reaction between methanol and oxygen to produce water and carbon dioxide.
A theoretical voltage generated in a DMFC single cell is approximately 1.2 V. However, the open circuit voltage under room temperature and atmospheric pressure conditions is 1 V or less and an actual operation voltage is approximately 0.3 to 0.5 V because there is a voltage drop due to activation over-potential and resistance over-potential. Thus, in order to generate a desirably high voltage, several single cells are stacked and electrically connected in series. The method stacking single cells in series is largely classified as a bipolar stack type and a monopolar cell pack type. The bipolar stack type is configured such that a single separator has both a positive (+) polarity and a negative (−) polarity and is suitably used for high power capacity. The monopolar cell pack type is configured such that a single separator has only a positive (+) or a negative (−) polarity and is suitably used for low power capacity.
According to the monopolar cell pack type, a plurality of single cells are arranged on an electrolyte membrane and then the respective single cells are connected in series, thereby considerably reducing the thickness and volume of fuel cell stack, realizing a lightweight, small-sized DMFC. In the monopolar cell pack type, the electrodes on the electrolyte membrane have all the same polarity, allowing fuel to be simultaneously supplied to all electrodes, thereby advantageously maintaining fuel concentrations of all the electrodes at a constant level.
However, in the monopolar cell pack, unlike the bipolar stack in which fuel supply and electrical connection are simultaneously established due to many graphite blocks each serving as a current collector and having a fuel flow field as a fuel supply path, it is difficult to simultaneously establish fuel supply and electrical connection. For this reason, when the contact between the current collector and anode or cathode is bad and a contact area is not wide, a current loss is generated due to contact resistance. Also, since efficient exhaustion of carbon dioxide as reaction byproducts is difficult to achieve, carbon dioxide bubbles permeate into a liquid fuel layer, thereby impeding fuel supply, and the bubbles produced on the electrode surface prevents fuel from moving to the catalyst layer, thereby noticeably deteriorating performance of electrodes.
To solve such drawbacks, a current collector plate enabling simultaneous fuel supply and current collection is necessary and such a current collector plate should be configured to maximize a contact area between the current collector plate and an electrode, thereby preventing a current loss due to contact resistance. Also, it is necessary to cause rapid exhaustion of carbon dioxide existing within the electrode by installing an appropriate exhaust path of carbon dioxide, thereby allowing fuel to be smoothly supplied to the catalyst layer.
Since a DMFC uses oxygen as a reactant gas, a DMFC cell pack should be configured such that its cathode for reduction directly contacts external air. However, when a DMFC cell pack is mounted on an electronic device to be used as a power source of the electronic device, an air inlet port formed on the external surface of the cell pack may be partially shielded at a connected area between the cell pack and the electronic device or the air inlet port may be shielded by user's body or according to use surroundings of the electronic device. In this case, since oxygen is not properly supplied to the shielded portion, electrode reactions do not occur thereat.
Another problem with the DMFC cell pack is that fuel cannot be evenly supplied to all electrodes. That is to say, since the fuel fed to the cell pack is in a liquid form, the fuel is gathered at one side of a fuel container by gravity when an electronic device equipped with a DMFC cell pack is used in a leaned state. Thus, the fuel is supplied only to electrodes located at the fuel gathered side but is not supplied to electrodes far from the fuel gathered side. Eventually, the required power output cannot be attained.
To overcome the problems, it is necessary to provide a cell pack having a structure capable of fully inducing external air thereinto to be evenly supplied to electrode surfaces, irrespective of a connected area between the cell pack and the electronic device or use surroundings of the electronic device. Also, separate means for preventing infiltration of external foreign matter or moisture must be provided. Further, there is a need for a fuel supply unit for evenly supplying fuel to all electrodes at any time irrespective of a use place or posture of the electronic device.